I. Field of the Invention
This invention relates generally to the casting of hollow ingots such as for use in the production of large diameter casings or pipes. More particularly, the disclosed invention relates to a method and apparatus for the semi-continuous casting of metallic hollow ingots and products resulting therefrom.
II. Background of the Related Art
Conventionally, the production of large diameter casings or pipes or rolled rings typically required the initial manufacture of a large diameter ingot followed by forging to produce a smaller diameter billet. The billet is then pierced to create a tubular preform and the tubular perform is then extruded to form the casing or pipe or rolled to form a ring. However, if it were possible to directly cast the tubular preform, significant downstream processing time and expense could be avoided.
Several attempts have been made to cast high-quality, large diameter hollow ingots. One approach involves inserting a water-cooled stationary mandrel into a molten pool. Once a sufficient amount of molten metal solidified onto the surface of the mandrel, the mandrel was withdrawn from the pool. After the solidified ingot was removed from the mandrel, the mandrel itself could be reintroduced into the molten pool and the process repeated.
Another attempt involves casting molten metal into a mold comprising a stationary core encapsulated by a crucible to form an annular space into which molten metal may be poured and allowed to solidify as described, for example, in U.S. Pat. No. 4,278,124 to Aso et al. (hereinafter “Aso”). In some embodiments, the interior of the core in Aso is cooled by forced induction, thereby providing control over the cooling rate at the interior wall of the cast hollow ingot.
Still another attempt involves adding a fixed amount of molten metal to a casting vessel. The vessel is then rotated and centrifugal forces drive the metal to the outer walls of the vessel. As the metal solidifies, a layer of the desired metal forms on the walls of the vessel, thereby producing a hollow ingot.
In yet another attempt, molten metal was introduced into an annular space formed by a stationary outer mold and stationary mandrel to facilitate continuous casting in a horizontal manner, as described in more detail is U.S. Pat. No. 4,456,054 to Henders.
However, all of the aforementioned attempts suffer from a number of problems including, but not limited to: the production of out-of-center internal holes, frequent breakouts at the inner mold surface, inconsistent dimensions, long cooling times, and slow casting rates.
Accordingly, there exists a need in the art for a more cost-effective technique for producing hollow ingots which is both sufficiently controllable and repeatable to be utilized as a commercial manufacturing process.